The field of the disclosure relates generally to electrical networks, and, more particularly, to tap changers in electrical networks and their methods of operation.
Many known electrical networks include electric power transformers configured to regulate voltages through the use of on-load tap changers. An on-load tap changing (OLTC) transformer has several connection points, so called “taps”, along at least one of its windings. With each of these tap positions a certain number of turns is selected. Since the output voltage of the OLTC transformer is determined by the turns ratio of the primary windings versus the secondary windings, the output voltage can be varied by selecting different taps. The tap position to connect to is determined by a suitable controller and tap selection is shifted through an OLTC device. Since high voltages are involved, and the taps are changed while the OLTC transformer is under load, each time a tap is changed, arcing occurs. Arcing may lead to deterioration of the associated materials, thereby tending to decrease the service life of the tap changer mechanisms. Therefore, it is typically desirable to shift taps as infrequently as possible.
However, it is not unusual to have multiple tap changes over a 24-hour period, especially with the increasing share of variable and intermittent distributed generation (DG) in electric networks. The operators of electric networks determine the tradeoff between the frequency and number of on-load tap changes with the subsequent wear on the tap changer and the quality of the voltage on the portion of the system maintained by the affected OLTC transformer.
Many known on-load tap changer controllers are configured to move the tap in an OLTC transformer automatically as a function of “raise” and “lower” commands to maintain the system voltage at a predetermined value, i.e., a constant voltage set-point. Typically, on-load tap changer controllers monitor the difference between the measured voltage at the on-load tap changer and the voltage set-point. The voltage step nature of on-load tap changer output requires a dead band or bandwidth around the voltage set-point to ensure stable operation. In prior art, this bandwidth is constant in width for all possible network states. A bandwidth that is defined to cover a smaller area around the voltage set-point keeps the measured voltage close to the set point voltage and thus leads to larger number of tap changes. On the other hand, a wider bandwidth allows the measured voltage to vary within a larger area and thus leads to less tap changes.
Many known electrical networks include a growing share of DG. Many types of DGs significantly increase the variability of the voltage on the portion of the system maintained by the affected OLTC transformer, thereby increasing the frequency of commanded tap changes. Moreover, with a significant portion of DG on one side of the transformer, i.e., typically the lower voltage downstream side, electric power flow through the OLTC transformer may be reversed, i.e., transmitted from the low voltage side to the high voltage side of the transformer. As such, the affected on-load tap changer controller needs to be configured to detect such a power flow reversal and still be able to ensure correct voltage regulation.
In response to the changing requirements of voltage regulation, systems that determine voltage set-points based on current network conditions have been devised. The current network state may be indicated e.g. by current or power flow measurements at the OLTC transformer. Although one measured current or power flow value does not explicitly correspond to one defined network state, i.e. different network states may result in the same measured current or power flow value, it does give an indication to the currently prevailing network state. In such systems, at times of large reverse power or current flow, which results in high network voltages, especially when DG is connected at remote feeder ends, a low voltage set-point is set. In contrast, during times of high demand by the loads and low network voltages, a higher voltage set-point is set. That way, the hosting capacity of the electrical network regarding distributed generation or e-mobility is increased by allowing variance in the voltage set-point based on network state.
In many of these variable set-point concepts, a set-point curve dependent on power flow or current over the tap changer is determined. The set-point curve may be linear in nature for a large portion. It has been observed that the linear nature of the set-point curve and a constant bandwidth around the voltage set-point may lead to frequent tap changes. In these concepts, the bandwidth remains of constant width around the set-point even in network states when a wider bandwidth may be permissible.
Hence, there is a need to define a bandwidth that is adjusted dependent on the current network state in order to ensure a minimum number of tap changes.